Anchoring device of spring steel and method for imparting the device with a bainitic structure



P 1970 HANS-DIETER SEGHEZZI ETAL 3,528,088

v ANCHORING DEVICE OF SPRING STEEL AND METHOD FOR IMPARTING' THE DEVICE WITH A BAINITIC STRUCTURE Filed Jan. 17, 1968 INVENTORS HA NS -DIETER SEGHEZZ KONRAD K055 TL IN mva m A TTORNEYS United States Patent 3,528,088 ANCHORING DEVICE OF SPRING STEEL AND METHOD FOR IMPARTING THE DEVICE WITH A BAINITIC STRUCTURE Hans-Dieter Seghezzi and Konrad Kiistlin, Vaduz, Liechtenstein, assignors to Hilti Aktieugesellschaft, Schaan, Liechtenstein Filed Jan. 17, 1968, Ser. No. 698,578 Claims priority, application Germany, Jan. 23, 1967, H 61,632 Int. Cl. C21d 1/20 U.S. Cl. 148143 5 Claims ABSTRACT OF THE DISCLOSURE Anchoring device composed of spring steel essentially consisting of Percent by Weight, between about Carbon 0.50-0.80 Silicon 0.70-2.00 Manganese 0.50-1.20 Chromium 0.50-1.20

phosphorus in an amount not exceeding 0.035 percent by weight and sulphur in an amount not exceeding 0.035 percent by weight, the spring steel of the anchoring device being further characterized by a predominantly bainitic structure and a hardness in the range of 55-5 8 HRc. The steel may also contain not more than about 0.5% by weight of molybdenum and/or vanadium. The application also discloses a method for imparting the steel composition of the anchoring device with the bainitic structure, the method comprising heating the anchoring device for about 10-30 minutes to a temperature in the range of 850-950 C. The thus heated device is then rapidly cooled to a temperature of about ZOO-280 C. and is maintained at this temperature for about 30-120 minutes, whereafter the device is cooled to room temperature.

SUMMARY OF THE INVENTION This invention generally relates to anchoring devices of steel and is particularly directed to such devices composed of a novel spring steel composition and to a method for imparting the composition with a predominantly bainitic structure.

The term anchoring devices as used herein is deemed to refer to bolts, studs, nails, and the like objects which are driven into solid materials such as concrete, structural iron and the like structural materials for connecting and assembly purposes. The driving of anchoring devices into the respective material is customarily accomplished by setting tools which may be manually operated and actuated in pneumatic or explosive manner.

In recent times, the need has arisen for driving such anchoring devices into extremely hard concrete and into structural steel of qualities known in the trade as St 42 and St 52. Experience has demonstrated that known anchoring devices of a customary hardness of up to 54 HRc cannot reliably be used and do not satisfactorily perform in connection with very hard concrete and structural steels of the indicated nature. In this connection it should be appreciated that in driving anchoring devices into materials of increasing stability and hardness, the hardness of the material of which the anchoring device is composed cannot simply be increased to a desired higher value since, as is well known, the tenacity or toughness of the material decreases with increasing hardness.

Accordingly, it is a primary object of this invention to provide anchoring devices which can reliably and readily be driven into structural material of extreme hardness.

C, about 0.50-0.80 Si, about 0.70-2.00 Mn, about 0.50-1.20 Cr, about 0.50-1.20 P, max 0.035 S, max. 0.035

Residue, iron.

the steel, furthermore, being characterized by having a predominantly bainitic structure and by a hardness, repro ducible in series, in the range of 55-58 HRc. In a preferred embodiment the steel of the anchoring device should have a completely bainitic structure.

The invention is based on the realization that, in respect to many steels of predominantly bainitic structure, the tenacity or toughness is greater, in a hardness of 55-58 H Rc, than it is with a martensitic structure.

Tests have indicated that anchoring devices with the above recited characteristics yield particularly advantageous results if the alloying metals of the steel compositions are contained in the composition in the following percentages:

Percent by weight 'Remainder, iron.

It has also been ascertained that an addition of not more than 0.5% by weight of molybdenum and/ or 0.5% by weight of vanadium imparts desirable characteristics to the anchoring device.

The desirable properties in respect to hardness and tenacity or toughness in anchoring devices made of the inventive steel composition, are only present if the bainitic structure of the steel is predominant, for example more than Such predominant bainitic structure is not obtained if the anchoring device is subjected to a conventional heat treatment. Accordingly, the invention provides for a particular method for imparting the steel composition of the anchoring device with the bainitic structure. This novel method resides in heating the anchoring device for about 10-30 minutes to a temperature in the range of 850-950 C. whereupon the device is rapidly cooled down to a temperature in the range of ZOO-280 C. The device is maintained within the latter temperature range for a period of about 30-120 minutes, whereafter cooling down to room temperature is efiected.

With a view to obtaining the desired bainitic structure by means of the inventive heat treatment, the alloy composition of the anchoring device to be subjected to the heat treatment should have a composition, particularly in respect to its content in carbide-forming elements, so that formation of carbides having a brittling effect is substantially avoided.

Anchoring devices made of the inventive steel composition are relatively inexpensive and therefore competitive. The steel composition may be processed into anchoring devices by cold working in economical manner. The anchoring devices may be subjected to subsequent zinc coating without negatively influencing the characteristics of the anchoring devices.

The invention will now be described by specific examples, it being understood, however, that these examples are given by way of illustration and not by way of limitation and that many changes may be effected Without affcct ing the scope and spirit of the invention as recited in the appended claims.

EXAMPLE I Nails are manufactured in conventional manner from a steel having the following composition:

Percent by Weight Carbon :6 Manganese 0.8 Silicon 1.8 Chromium 0.5 Molybdenum 0.4

the remainder consisting of iron. The thus produced nails are heated for 15 minutes to a temperature of 910 C. and are then cooled down to a temperature of 250 C. by dippin'g them into a salt 'bath. The nails were maintained at the 250 C. temperature for 90 minutes and were allowed to cool down to room temperature. Tests indicated that the nails thusobtained had a hardness of 56 HRc. The martensitic moiety amounted to 7% by volume, the remainder of the structure being of bainitic nature.

EXAMPLE II This example compares for the inventive anchoring devices the tenacity or toughness obtained with respectively martensitic and predominantly bainitic structure at different hardness values. The tenacity or toughness was measured as a function of impact work of a smooth round sample having the following composition:

Percent by weight C 0.6 Cr 0 5 Mn 0.8 Mo 0.4 Si 1:8

Residue, iron.

Impact work kpcmJmm.

It is clear from the table that for the indicated purpose of predominantly bainitic structure of the examined steel is significantly superior as compared to the steel of martensitic structure. It follows that the steel, in addition to its hardness, must have predominantly bainitic structure in order to have the necessary tenacity or toughness.

The single drawing shows an elevation of an anchoring device made from the inventive steel composition.

While a specific embodiment of the invention has been shown and described in detail to illustrate the application of the inventive principles, it will be understood that the invention may be embodied otherwise without departing from such principles.

4 What is claimed is: 1. An anchoring device composed of alloy spring steel, said steel essentially consisting of Percent by weight, between about Carbon 0.50-0.80 Silicon 0.70-2.00 Manganese 0.50-1.20 Chromium 0.50-1.20

phosphorus in an amount not exceeding 0.035 percent by Weight and sulphur in an amount not exceeding 0.035 percent by weight, the remainder being iron, the spring steel of said device being further characterized by a predominantly bainitic structure and a hardness in the range of 55-58 HRc.

2. An anchoring device as claimed in claim 1, wherein the spring steel further comprises not more than about 0.5 percent by weight of molybdenum and/or not more than about 0.5 percent of vanadium.

3. An anchoring device as claimed in claim 1, wherein the steel has the following composition:

Percent by weight C, about a 0.60-0.70 Si, about 1.4-1.6 Mn, about 0.55-0.6-5 Cr, about 0.60-0.80 P, max 0.025 S, max 0.025

Remainder, iron 4. A method of imparting predominantly bainitic structure to an anchoring device composed of a spring steel essentially consisting of Percent by weight, between about Carbon 0.50-0.80 Silicon 0.70-2.00 Manganese 0.50-1.20 Chromium 0.50-1.20

phosphorus in an amount not exceeding 0.035 percent by weight, and sulphur in an amount not exceeding 0.035 percent by weight, the remainder being iron which comprises heating the anchoring device for about 10-30 minutes to a temperature of about 850-950 C, then rapidly cooling the device to a temperature of about 200-280" C., maintaining the device Within said last-mentioned temperature range for a period of about 30-120 minutes and thereafter cooling the device to room temperature.

5. A method as claimed in claim 4, wherein the device additionally contains not more than 0.5 percent by weight of molybdenum and/or 0.5 percent by weight of vanadium. 

